1. Field of the Invention
The present invention relates to a process for fabricating a bipolar transistor. More specifically, it relates to a process for fabricating a self-aligned bipolar transistor with a reduced collector-base junction capacitance and base resistance.
2. Description of the Related Art
A typical bipolar transistor comprises a semiconductor substrate having an n-type epitaxial semiconductor layer at the surface side thereof, in which an n-type region is isolated by an pn-junction or an insulating region to constitute a collector region. A base region is a p-type doped region formed in the collector region at the surface of the epitaxial layer, and an emitter region of an n.sup.+ -type doped region is formed in the base region. An emitter electrode is formed on the surface of the n.sup.+ -type emitter region and a base electrode is formed on the surface of the p-type base region.
In this typical bipolar transistor, the base region must have a considerably larger area than that of the emitter, in order to provide insulation between the emitter and base electrodes based on a mask alignment. However, preferably the area of the base region should be as small as possible, since an essential active portion of the base region necessary for transistor operation is only the portion underlying the emitter region. The other portion of the base region, which is used only for taking out a base electrode, increases the base resistance and the collector base junction capacitance, decreasing the operating speed of the transistor, and prevents an increase of the integration degree of the device.
In order to alleviate the above problems, a bipolar transistor has been proposed in which an intrinsic base region is formed to have an area essentially necessary for transistor operation and a base-electrode taking-out portion is made of polycrystalline silicon (T. Nakamura et al., "Self-Aligned Transistor with Sidewall Base Electrode", IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, No. 4, April 1982, 596-600). In this transistor, the area of the collector-base junction is small so that the collector-base junction capacitance is reduced and the operating speed of the transistor is increased. However, this device is fabricated by a process which is undesirably complex.